Fluid actuators and particularly pneumatic actuators are in widespread use in both commercial and industrial applications. Oftentimes, it is desirable to operate such an actuator at a predetermined rate such as, for example, when the actuator is employed to set a valve controlling high pressure bleed air from the compressor discharge section of a gas turbine engine. In such applications, actuator operation at less than desired slew rates would militate against accurate control of airflow through the actuated valve. On the other hand, slew rates greater than desired could overly stress the valve and actuator structures.
In the prior art, attempts have been made to control pneumatic actuator slew rates by controlling airflow to the actuator by means of servo fluid pressure control with a single orifice. However, it has been found that generally, a single orifice either does not allow sufficient flow for operation of the actuator at a desired speed or, provides excessive flow which cause the actuator to operate at slew rates higher than desired. While variable rate fluid motors are known in the art, such devices are generally not adaptable for use in actuators of the type discussed herein.